Crank mechanism

ABSTRACT

A crank mechanism for an internal combustion engine comprises a cylinder (1), a piston (2) reciprocable within the cylinder, and a rotatable shaft (4). The piston (2) is a drivable connection with the shaft (4) via a connecting rod (3), a drive ring (5) and a torque lobe (6). The connecting rod (3) is pivotally fixed to the piston (2), and the drive ring (5) is rigidly attached to the free end of the connecting rod (3). The torque lobe (5) is a circulate plate eccentrically mounted on the shaft (4) for rotation therewith about the axis thereof. The drive ring (5) is an annular sleeve which is a rotatable sliding fit around the rim of the torque lobe (6). The axis of the piston (2) is offset with respect to the center of the output shaft (4), whereby rectilinear movement of the piston (2) is converted to rotary movement of the torque lobe (6) or vice versa.

This application is a continuation-in-part of U.S. patent applicationSer. No. 08/165,969, filed Dec. 10, 1993, and now abandoned, which is acontinuation-in-part of U.S. patent application Ser. No. 07/910,186,filed Jul. 16, 1992, now U.S. Pat. No. 5,297,448.

This invention relates to a crank mechanism for an internal combustionengine.

A conventional internal combustion engine employs a crankshaft toconvert the reciprocating motion of the piston(s) into output torque topropel a vehicle or to act upon any other load. The crankshaft isinefficient in terms of converting the power available from the fuelcombustion into usable output torque. This is because combustion of thefuel/air mixture takes place at approximately the top dead centre (TDC)position of the piston. Not only the crankpin, but also the crankshaftmain bearings, are consequently subjected to periodic heavy stresses.What is of greater significance, however, is that, with an internalcombustion engine provided with conventional drive gear, the ignitedfuel/air pressure forces cannot produce torque when the piston is eitherat TDC or bottom dead centre (BDC), as the connecting rod and thecrankpin are practically in a straight line so that there is no forcecomponent tangential to the crank circle. This results in most of theavailable energy being lost as heat. The torque necessary to carry thecrankshaft through these two dead centre positions is supplied by theinertia of the flywheel of the engine. Moreover, by the time thecrankshaft has rotated through almost 90° beyond TDC, where the turningmoment is a maximum, the piston force is greatly reduced, so that theresulting torque is relatively small.

My International patent application WO91/10848 describes a crankmechanism for an internal combustion engine, the crank mechanismcomprising a cylinder, a piston reciprocable within the cylinder, and arotatable shaft. The piston is in drivable connection with the shaft viaa connecting rod, a drive ring and a torque lobe. The connecting rod ispivotally fixed to the piston, and the drive ring is pivotally attachedto the free end of the connecting rod. The torque lobe is a circularplate eccentrically mounted on the shaft for rotation therewith aboutthe axis thereof. The drive ring is slidable along, but restrained to,the rim of the torque lobe, and the drive ring is constrained to movearound a closed path whereby rectilinear movement of the piston isconverted to rotary movement of the torque lobe or vice versa. The drivering is provided with cam follower means which engage with associatedcam track means adjacent to the torque lobe, the cam follower means andthe cam track means constituting means for constraining the drive ringto move round the closed path.

A disadvantage of this crank mechanism is the need for the cam trackmeans and the cam follower means, which complicate the design. Moreover,this mechanism requires an anti-reverse device for preventing the torquelobe from rotating in the reverse direction at BDC at the end of theinduction stroke.

The present invention provides a crank mechanism for an internalcombustion engine, the crank mechanism comprising a cylinder, a pistonreciprocable within the cylinder, and a rotatable shaft, the pistonbeing in drivable connection with the shaft via a connecting rod, adrive ring and a torque lobe, the connecting rod being pivotally fixedto the piston, and the drive ring being rigidly attached to the free endof the connecting rod, wherein the torque lobe is a circular plateeccentrically mounted on the shaft for rotation therewith about the axisthereof, wherein the drive ring is an annular sleeve which is arotatable sliding fit around the rim of the torque lobe, and wherein theaxis of the piston is offset with respect to the centre of the outputshaft whereby rectilinear movement of the piston is converted to rotarymovement of the torque lobe or vice versa.

This mechanism has the advantages of not requiring either cam track/camfollower means or an anti-reverse device, and so is a simpler and morereliable mechanism than my earlier crank mechanism, whilst still havingimproved torque characteristics.

The invention also provides a crank mechanism for an internal combustionengine, the crank mechanism comprising a plurality of cylinders, arespective piston reciprocable within each of the cylinders, and arotatable shaft, each of the pistons being in drivable connection withthe shaft via a respective connecting rod, a respective drive ring and arespective torque lobe, each connecting rod being pivotally fixed to theassociated piston, and each drive ring being rigidly attached to thefree end of the associated connecting rod, wherein each torque lobe is acircular plate eccentrically mounted on the shaft for rotation therewithabout the axis thereof, wherein each drive ring is an annular sleevewhich is a rotatable sliding fit around the rim of the associated torquelobe, and wherein the axis of each piston is offset with respect to thecentre of the output shaft whereby rectilinear movement of the pistonsis converted to rotary movement of the torque lobe or vice versa.

Preferably, the or each drive ring is a rolling fit on the rim of theassociated torque lobe. In this case, the or each drive ring is mountedon the rim of the associated torque lobe by means of a respectiverolling element bearing, whereby the rolling elements and the torquelobe rotate in the same direction, thereby increasing the turning momentof the torque lobe and hence that of the crank mechanism.

Advantageously, the axis of the or each piston is offset from the axisof the output shaft by a distance equal to substantially half the strokeof the associated piston.

In a preferred embodiment, the or each connecting rod is constituted bya main connecting rod and at least one auxiliary connecting rod, the oreach auxiliary connecting rod being slidably fixed to the mainconnecting rod for axial movement relative thereto, the associatedpiston being fixed to the or each auxiliary connecting rod, and the mainconnecting rod being fixed to the associated drive ring. Conveniently, arespective pair of auxiliary connecting rods are associated with the oreach main connecting rod, the auxiliary connecting rods of the or eachpair being positioned one on each side of the associated main connectingrod and being slidably fixed thereto by axial slots formed in the mainconnecting rod and pins projecting from the auxiliary connecting rodsand passing through the slots. The or each auxiliary connecting rod maybe associated with a respective cam fixed to the associated torque lobe.Where there are two auxiliary connecting rods associated with the oreach main connecting rod, the two associated cams are fixed to oppositesides of the associated torque lobe.

Preferably, the or each cam is formed with a cam drive face whichengages with the free end of the associated auxiliary connecting rodover the first 90° of movement of the associated piston during its powerstroke, thereby applying a turning moment to said cam and hence to saidcam and hence to said torque lobe. The or each cam may be formed with areturn cam face which engages the free end of the associated auxiliaryconnecting rod during the movement of the associated piston in itsexhaust stroke, and over the last 90° of movement of the said pistonduring its compression stroke.

Advantageously, the mechanism further comprises a respective springassociated with the or each auxiliary connecting rod, the or each springbeing effective to hold the associated auxiliary connecting rod out ofcontact with its cam during the induction stroke of the associatedpiston.

The addition of the auxiliary connecting rod(s) arranged to run parallelto the main connecting rod leads to a further improvement in the crankmechanism. This is because the or each piston imparts high turningmoments via the associated cam(s) to the output shaft for the first 90°travel on the power stroke. The turning moment is closely matched to thepower curve, thereby maximising the output power. The mechanism thenreverts to the arrangement previously described for the second 90°travel of the power stroke, utilising the advantages of that system. Thecam arrangement which includes the spring(s) has the advantage of asmooth action, as well as resetting the auxiliary connecting rods. Italso creates a situation where resonance can occur provided by therotating mass and spring compliance, such that the system will have avery high efficiency at a particular rotational speed.

Conveniently, there are six cylinders arranged in two banks of threecylinders, the cylinders in each bank being in a flat radialconfiguration. In this case, the three torque lobes associated with eachbank of three cylinders may all be fixed to the output shaft in such amanner that the lines joining the centre of the output shaft to thecentres of said torque lobes are angled to one another by 120°.

The invention will now be described in greater detail, by way ofexample, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic part-sectional elevation of a simplified form ofinternal combustion engine which illustrates the principle of theinvention;

FIG. 2 is a schematic, partially broken away, perspective view of theengine of FIG. 1;

FIGS. 3a to 3d are diagrams illustrating different stages of theoperation of the engine of FIGS. 1 and 2;

FIGS. 4a to 4e are diagrams illustrating how improved torque is achievedwith the engine of FIGS. 1 and 2;

FIG. 5 is a perspective view of an internal combustion engineincorporating a modified form of the crank mechanism of FIGS. 1 and 2;

FIG. 6 is a side elevation of the crank mechanism of FIG. 5, and showsthe connecting rod in more detail; and

FIG. 7 is a plan view of a practical form of internal combustion engineincorporating the crank mechanism of FIGS. 1 and 2.

Referring to the drawings, FIGS. 1 and 2 show a simple form of internalcombustion engine. The engine has a single cylinder 1 in which a piston2 is reciprocable. The piston-and-cylinder arrangement 1, 2 has aconventional valve arrangement indicated generally by the reference V,and can be powered by petrol, diesel or any other acceptable fuel. Aconnecting rod 3 is pivotally fixed to the piston 2, the connecting rodrotatably driving an output shaft 4 via a drive ring 5 and a torque lobe6. The connecting rod 3 is rigidly fixed to the drive ring 5. The torquelobe 6 is constituted by a circular plate, which is eccentricallymounted on the shaft 4. As shown best in FIGS. 1 and 3, the axis of thepiston 2 is offset with respect to the centre of the output shaft 4,that is to say the line of action of the piston does not pass throughthe centre of the output shaft.

As shown best in FIG. 2, the drive ring 5 is an annular sleeve which canslide round the torque lobe 6 as it rotates. The drive ring 5 isattached to a ring (not shown) fixed to the outer race 8 of a ballbearing 7 whose inner race 9 is fixed to the peripheral edge of thetorque lobe 6. A roller bearing could be used in place of the ballbearing.

The operation of the single cylinder engine of FIGS. 1 and 2 will now bedescribed with reference to FIGS. 3a to 3d. The operating cycle is bestunderstood by describing the operation of a four-stroke cycle startingwith the piston 2 at TDC. In this position (see FIG. 3a), the piston 2has just completed its compression stroke, ignition having taken placejust before TDC. The expanding gases formed by the ignition explosionforce the piston 2 to descend rapidly within the cylinder 1, whereby theconnecting rod 3 and the drive ring 5 force the torque lobe 6 to rotateto the position shown in FIG. 3b. This movement of the piston 2 is thepower stroke of the engine. Further rotation of the torque lobe 6carries the piston 2 past BDC (see FIG. 3c), where appropriate action ofthe valve arrangement (not shown in this figure) causes the spent gasesto be exhausted from the engine. The momentum of the torque lobe 6and/or a flywheel (not shown) on the output shaft 4 carries the piston 2upwards through the position shown in FIG. 3d and towards TDC. After thepiston 2 is carried past TDC, the momentum of the flywheel forces thepiston and the drive ring 5 down into the induction stroke (see FIGS. 3aand 3b).

Once the piston 2 has passed BDC, it is carried upwards into thecompression stroke (see FIGS. 3c and 3d ) by the momentum of the torquelobe 6 and/or the flywheel. The operating cycle then starts again withthe piston 2, the drive ring 5 and the torque lobe 6 in the positionsshown in FIG. 3a.

During the four strokes, the drive ring 5 is constrained to follow therotation of the torque lobe 6 twice (see FIG. 6a), the TDC position ofthe piston 2 being offset from the axis of the shaft 4 by a distanceequal to half the length of the stroke of the piston, or by a distancechosen to match the characteristics of the input energy. This enhancesthe torque produced by the engine by increasing the moment of thedescending piston 2 about the axis of rotation (the axis of the shaft4), in a manner described below with reference to FIGS. 4a to 4e.

FIGS. 4a to 4e show the principle underlying the derivation of theimproved torque of the engine of FIGS. 1 and 2. FIG. 4a shows aconventional ball bearing 10, this bearing normally rotating about itsown geometric axis 11 with either its inner ring 12 or its outer ring 13held. However, if an external force F (see FIG. 4b) is applied to theouter ring 13, thereby causing the bearing 10 to rotate, this forceproduces a two-link mechanism whereby the balls 14 of the bearing 10 arerotated about their own axes in the direction opposite to that in whichthe inner ring 12 rotates, such that the balls rotate on the outer ring(which is not free to move). The combined effect of the external force Fand the rotation of the balls 14 has the effect of moving the centre ofthe turning circle of the ball cage (not shown) by an amount x equal tothe radius of the balls (see FIG. 4c).

Applying this theory to a conventional crank mechanism 15 (see FIG. 4d),it can be seen that the crank pin 16 rotates in the direction of thearrow A, this direction being opposite to the direction B of rotation ofthe output shaft 17 of the mechanism 15. Thus, the output shaft 17rotates in the opposite direction to that (C) in which the balls of thebearing rotate. The resultant shift (not shown) of the centre of theturning circle of the ball cage, therefore, has the effect of reducingthe overall turning moment of the output shaft by the ratio of ballradius to crank throw radius (that is to say the distance between thecentres of the crank pin 16 and the output shaft 17). If, however, thisprinciple is applied to the torque lobe mechanism of FIGS. 1 and 2, theoutput shaft 4 (see FIG. 4e) rotates in the same direction B as that (A)in which the torque lobe 6 rotates. Thus, the output shaft 4 rotates inthe same direction as that (C) in which the balls of the bearing 7rotate. The shift (not shown) of the centre of the turning circle of theballs will, therefore, assist the overall turning moment of the outputshaft 4 by the ratio of ball radius to centre-of-lobe tocentre-of-output shaft distance (that is to say the turning moment). Asthe piston 2 is offset with respect to the centre of the output shaft 4,this assists the overall improvement by increasing the force applied tothe torque lobe 6 by the inverse cosine (secant) of the angle betweenthe axes of the connecting rod 3 and the piston.

Clearly, the larger the bearing ball diameter for a given torque lobeturning moment the greater the overall power output from the enginewithin engineering limits.

FIG. 5 shows an internal combustion engine incorporating a modified formof the crank mechanism of FIGS. 1 and 2. This engine has a furtherincrease in turning moment when compared with the engine whichincorporates the crank mechanism of FIGS. 1 and 2. This modified crankmechanism is similar to that of FIGS. 1 and 2, so like referencenumerals will be used for like parts, and only the modifications will bedescribed in detail. Thus, the connecting rod 3 of the FIG. 5 embodimentis provided with a pair of auxiliary connecting rods 21 which areconstrained to move parallel to the main connecting rod by pins 22a and22b projecting from the auxiliary connecting rods and passing throughaxial slots 23a and 23b formed in the main connecting rod. The upper pin22a is a gudgeon pin connecting the auxiliary connecting rods 21 to thepiston 2. The lower pin 22b constrains the auxiliary connecting rods 21to be parallel to the main connecting rod 3 which is in rigid connectionwith the drive ring 5.

Two external cams 24 are provided, being fixed to the torque lobe 6 onopposite sides thereof. The cams 24 each have a curved drive face 24aand a curved return face 24b which meet at an apex portion 24c, theportion of the cam between the drive and return faces constituting anon-camming face 24d. As shown in FIG. 6, the apex portion 24c iscoincident with the axis of the movement of the piston 2 at TDC. Thepiston 2 is at TDC at the commencement of its power stroke. Pressure inthe cylinder 1 will cause the piston 2 to move down, thereby driving theauxiliary connecting rods 21 along the inclined drive faces 24a of thecams 24. A considerable turning moment will be exerted on these camfaces 24a which, in turn, will rotate the torque lobe 6, the cams 24,the bearing 7 and the drive ring 5.

When the torque lobe 6 and attachments are at approximately 90° afterTDC, the pins 22a and 22b will reach the lower ends of the slots 23a and23b. Here, the cam faces 24d have been relieved such that the auxiliaryconnecting rods 21 no longer apply force to the cams 24. This force isnow transferred to the main connecting rod 3 by the engagement of thepins 22a and 22b with the lower ends of the slots 23a and 23b. Themechanism now behaves for the remaining 90° of the power stroke as themechanism shown in FIGS. 1 and 2. This transfer of force from the cams24 to the drive ring 5 and the torque lobe 6 is necessary because theturning moment exerted by the auxiliary connecting rods 21 at this point(90° after TDC) reduces below that which can be exerted by the mainconnecting rod 3. This maximises the overall turning moment on the powerstroke.

On completion of the power stroke, the piston 2 is required to bereturned to its starting position (with the pins 22a and 22b at theupper ends of the slots 23a and 23b) during the exhaust stroke. Thereturn faces 24b of the cams 24 provide this function, enabling theauxiliary connecting rods 21 to be raised by the cams 24 such that allare returned to their same positions as at the commencement of the powerstroke. As the engine continues to rotate past TDC, it commences itsinduction stroke, the inlet valve is open and the piston is descending.A partial vacuum is created in the cylinder 1 holding the auxiliaryconnecting rods 21 and the piston 2 such that the pins 22a and 22b areheld at the upper ends of the slots 23a and 23b. It is necessary to holdthe piston 2 and the auxiliary connecting rods 21 in this position toprevent physical contact of the auxiliary connecting rods with the cams24 which would otherwise be caused by the inertia gained by these rodsand the piston, their having been accelerated over the first 90° of theinduction stroke. Springs 25 (one for each auxiliary connecting rod)provide the function of holding the auxiliary connecting rods 21 and thepiston 2 in the positions where the pins 22a and 22b are at the upperends of the slots 23a and 23b for the 180° rotation of the inductioncycle. This also ensures that the induction stroke is not increased,which could occur, due to the descent of the piston 2 and the auxiliaryconnecting rods 21 had the springs 25 not been provided.

On completion of the induction stroke, the compression stroke commences.As the torque lobe 6 passes through BDC, the increasing compression inthe cylinder 1 will cause the auxiliary connecting rods 21 and thepiston 2 to descend gradually. By about 90° before TDC, the auxiliaryconnecting rods 21 will touch the return faces 24b of the cams 24 andthereafter will be lifted on these cam faces to TDC for the next powerstroke.

Clearly the inclusion of the springs 25 in this mechanism will cause thestoring of some energy during the power stroke, but this energy will bereturned to the system on the exhaust stroke as the springs return totheir rest positions.

Because the auxiliary connecting rods 21 and the piston 2 are free tomove relative to the main connecting rod 3, the power stroke isincreased by 50%. This extra movement does not occur on the inductionstroke due to the springs 25 holding the auxiliary connecting rods 21with the pins 22a and 22b at the upper ends of the slots 23a and 23b.This means that the pressure that would have been released in aconventional engine when the exhaust valve opens at approximately20°-30° before BDC, is able to be utilised to create turning moment,thereby expending more energy in output as opposed to exhaust heat.

Clearly, the engine of FIGS. 1 and 2 produces an average torque which issubstantially larger than that of the conventional engine, with theengine of FIG. 5 being even further improved. To summarise, the torquelobe arrangement of the crank mechanism of the present invention, inutilising the turning moment of the balls, and by the piston offset moreclosely matching the `moment curve` to the `pressure curve`, greatlyenhances the mechanical efficiency of the process of convertingreciprocating motion to output torque.

FIG. 7 shows a practical form of internal combustion engineincorporating the crank mechanism of FIGS. 1 and 2, this engine havingtwo banks (only one of which can be seen--the other being directlybehind) of three cylinders 31, each acting on a respective torque lobe36. Each cylinder 31 has a respective piston 32, connecting rod 33 anddrive ring 35, and its torque lobe 36 is eccentrically mounted on acommon output shaft 34. A respective drive ring 35 is attached to a ring(not shown) fixed to the outer race of a respective roller bearing 37whose inner race is fixed to the peripheral edge of the associatedtorque lobe 36. Each drive ring 35 is rigidly attached to its associatedconnecting rod 33. The torque lobes 36 of each bank are rigidly attachedto each other and to the output shaft 34 for rotation therewith. Eachbank of cylinders 31 is a flat radial configuration with the outputshaft 34 acting vertically downwards into a gear box (not shown). Thesix cylinders 31 are arranged to fire in the order 1 4 2 5 3 6. Thiswould require a distributor (not shown) to initiate a spark on bothexhaust and compression strokes, and would enable the distributor to bedriven directly from the output shaft 34 with no gearing. The wholearrangement, if applied to a four-stroke engine, would need inertia(provided by the lobe masses) to conserve momentum. Balancing for theoscillating mass of the pistons 32 can be provided by arranging the twobanks of cylinders 31 such that the cylinders diametrically opposite oneanother fire together.

Obviously, the engine of FIG. 7 could, alternatively, incorporate thecrank mechanism of FIGS. 5 and 6 in place of the crank mechanism ofFIGS. 1 and 2.

A particular advantage of either of the crank mechanisms described aboveis that, by improving the overall efficiency of the associated engine,the loss of energy in the form of heat is substantially reduced, therebyreducing (or even eliminating) the need for engine cooling. This willlead to further improvements in the efficiency with which an associatedvehicle is propelled, as the overall weight of the vehicle can bereduced by removing the need for components such as a cooling fan, awater pump, a water jacket and a radiator.

Each of the crank mechanisms described above has a reduced crankshaftlength compared with a conventional crankshaft, and so can be used inany of the presently accepted multi-cylinder arrangements, that is tosay radial, straight, flat or V-formation. This type of mechanism canalso be used with any appropriate number of cylinders.

It would also be possible to operate any of the engines described aboveon a two-stroke cycle. The crank mechanism of the invention could alsobe incorporated in other forms of reciprocating engine such as a steamengine. Obviously, such a crank mechanism could also form part of apump, for converting rotary motion of an input shaft to reciprocatorymotion of a piston within a cylinder. Such a pump would have a muchgreater efficiency than a conventionally-cranked device. Indeed, thecrank mechanism of the invention can be used with considerable advantagein any form of reciprocating/rotary or rotary/reciprocating arrangement,in any single or multi-cylinder configuration, and with any suitablefuel.

The engine of FIGS. 5 and 6, which uses cams and springs, would beparticularly suitable for a constant-speed engine arrangement running atits "resonant frequency", and may be applied as an "on-board" engine foran electrically-propelled car where the engine can be used veryefficiently to change the car batteries whilst the car is running.

I claim:
 1. A crank mechanism for an internal combustion engine, thecrank mechanism comprising a cylinder, a piston reciprocable within thecylinder, and a rotatable shaft, the piston being in drivable connectionwith the shaft via a connecting rod, a drive ring and a torque lobe, theconnecting rod being pivotally fixed to the piston, and the drive ringbeing rigidly attached to the free end of the connecting rod, the torquelobe being a circular plate eccentrically mounted on the shaft forrotation therewith about the axis thereof, the drive ring being anannular sleeve which is rotatable around the rim of the torque lobe,whereby rectilinear movement of the piston is converted to rotarymovement of the torque lobe or vice versa, the connecting rod beingconstituted by a main connecting rod and at least one auxiliaryconnecting rod, the or each auxiliary connecting rod being slidablyfixed to the main connecting rod for axial movement relative thereto,the piston being fixed to the or each auxiliary connecting rod, and themain connecting rod being fixed to the drive ring, the or each auxiliaryconnecting rod being associated with a respective cam fixed to thetorque lobe.
 2. A crank mechanism as claimed in claim 1, wherein thedrive ring is a rolling fit on the rim of the torque lobe.
 3. A crankmechanism as claimed in claim 2, wherein the drive ring is mounted onthe rim of the torque lobe by means of a rolling element bearing,whereby the rolling element bearing and the torque lobe rotate in thesame direction, thereby increasing the turning moment of the torque lobeand hence that of the crank mechanism.
 4. A crank mechanism as claimedin claim 1, wherein the axis of the piston is offset from the axis ofthe output shaft by a distance equal to substantially half o\the strokeof the associated piston.
 5. A crank mechanism as claimed in claim 1,wherein a respective pair of auxiliary connecting rods are associatedwith the main connecting rod, the auxiliary connecting rods of the pairbeing positioned one on each side of the main connecting rod and beingslidably fixed thereto by axial slots formed in the main connecting rodand pins projecting from the auxiliary connecting rods and passingthrough the slots.
 6. A crank mechanism as claimed in claim 1 whereinthe auxiliary connecting rod is associated with a cam fixed to thetorque lobe.
 7. A crank mechanism as claimed in claim 1, wherein arespective pair of auxiliary connecting rods are associated with themain connecting rod, the auxiliary connecting rods of the pair beingpositioned one on each side of the main connecting rod and beingslidably fixed thereto by axial slots formed in the main connecting rodand pins projecting from the auxiliary connecting rods and passingthrough the slots, the two cams associated with torque lobe being fixedto opposite sides of said torque lobe.
 8. A crank mechanism as claimedin claim 7, wherein each cam is formed with a cam drive face whichengages with the free end of the associated auxiliary connecting rodover the first 9 ° of movement of the associated piston during its powerstroke, thereby applying a turning moment to said cam and hence to saidtorque lobe.
 9. A crank mechanism as claimed in claim 8, wherein eachcam is formed with a return cam face which engages the free end of theassociated auxiliary connecting rod during the movement of theassociated piston in its exhaust stroke, and over the last 90° ofmovement of the said piston during its compression stroke.
 10. A crankmechanism as claimed in claim 7, wherein each cam is formed with a camdrive face which engages with the free end of the associated auxiliaryconnecting rod over the first 90° of movement of the associated pistonduring its power stroke, thereby applying a turning moment to said camand hence to said torque lobe.
 11. A crank mechanism as claimed in claim10, wherein each cam is formed with a return cam face which engages thefree end of the associated auxiliary connecting rod during the movementof the associated piston in its exhaust stroke, and over the last 90° ofmovement of the said piston during its compression stroke.
 12. A crankmechanism as claimed in claim 11, further comprising a respective springassociated with each auxiliary connecting rod, each spring beingeffective to hold the associated auxiliary connecting rod out of contactwith its cam during the induction stroke of the associated piston.
 13. Acrank mechanism as claimed in claim 11, further comprising a springassociated with each auxiliary connecting rod, each spring beingeffective to hold the associated auxiliary connecting rod out of contactwith its cam during the induction stroke of the pistol.
 14. A crankmechanism for an internal combustion engine, the crank mechanismcomprising a plurality of cylinders, a respective piston reciprocablewithin each of the cylinders, and a rotatable shaft, each of the pistonsbeing in drivable connection with the shaft via a respective connectingrod, a respective drive ring and a respective torque lobe, eachconnecting rod being pivotally fixed to the associated piston, eachdrive ring being rigidly attached to the free end of the associatedconnecting rod, each torque lobe being a circular plate eccentricallymounted on the shaft for rotation therewith about the axis thereof, eachdrive ring being an annular sleeve which is rotatable around the rim ofthe associated torque lobe, whereby rectilinear movement of theassociated piston is converted to rotary movement of the associatedtorque lobe or vice versa, each connecting rod being constituted by amain connecting rod and at least one auxiliary connecting rod, the oreach auxiliary connecting rod being slidably fixed to the associatedmain connecting rod for axial movement relative thereto, each pistonbeing fixed to the or each associated auxiliary connecting rod, and eachmain connecting rod being fixed to the associated drive ring, andwherein the or each auxiliary connecting rod is associated with arespective cam fixed to the associated torque lobe.
 15. A crankmechanism as claimed in claim 14, wherein the or each drive ring is arolling fit on the rim of the associated torque lobe.
 16. A crankmechanism as claimed in claim 15, wherein each drive ring is mounted onthe rim of the associated torque lobe by means of a respective rollingelement bearing, whereby the rolling element bearings and the torquelobe rotate in the same direction, thereby increasing the turning momentof the torque lobe and hence that of the crank mechanism.
 17. A crankmechanism as claimed in claim 14, wherein the axis of each piston isoffset from the axis of the output shaft by a distance equal tosubstantially half the stroke of the associated piston.
 18. A crankmechanism as claimed in claim 14, wherein a respective pair of auxiliaryconnecting rods are associated with each main connecting rod, theauxiliary connecting rods of each pair being positioned one on each sideof the associated main connecting rod and being slidably fixed theretoby axial slots formed in the main connecting rod and pins projectingfrom the auxiliary connecting rods and passing through the slots.
 19. Acrank mechanism as claimed in claim 14, wherein each auxiliaryconnecting rod is associated with a respective cam fixed to theassociated torque lobe.
 20. A crank mechanism as claimed in claim 14,wherein the two cams associated with each torque lobe being fixed toopposite sides of said torque lobe.
 21. A crank mechanism as claimed inclaim 14, wherein there are six cylinders arranged in two banks of threecylinders, the cylinders in each bank being in a flat radialconfiguration.
 22. A crank mechanism as claimed in claim 21, wherein thethree torque lobes associated with each bank of three cylinders are allfixed to the output shaft in such a manner that the lines joining thecentre of the output shaft to the centres of said torque lobes areangled to one another by 120°.
 23. A crank mechanism for an internalcombustion engine, the crank mechanism comprising a cylinder, a pistonreciprocable within the cylinder, and a rotatable shaft, the pistonbeing in drivable connection with the shaft via a connecting rod, adrive ring and a torque lobe, the connecting rod being pivotally fixedto the piston, and the drive ring being rigidly attached to the free endof the connecting rod, the torque lobe being a circular plateeccentrically mounted on the shaft for rotation with the shaft about theaxis thereof, the drive ring being an annular sleeve which has arotatable sliding fit around the rim of the torque lobe, the axis of thepiston being offset with respect to the center of the output shaftwhereby rectilinear movement of the piston is converted to rotarymovement of the torque lobe or vice versa, the connecting rod beingconstituted by a main connecting rod and at least one auxiliaryconnecting rod, the or each auxiliary connecting rod being slidablyfixed to the main connecting rod for axial movement relative thereto,the piston being fixed to the or each auxiliary connecting rod, and themain connecting rod being fixed to the drive ring, and wherein the oreach auxiliary connecting rod is associated with a respective cam fixedto the torque lobe.
 24. A crank mechanism for an internal combustionengine, the crank mechanism comprising a plurality of cylinders, arespective piston reciprocable within each of the cylinders, and arotatable shaft, each of the pistons being in drivable connection withthe shaft via a respective connecting rod, a respective drive ring and arespective torque lobe, each connecting rod being pivotally fixed to theassociated piston, and each drive ring being rigidly attached to thefree end of the associated connecting rod, each torque lobe being acircular plate eccentrically mounted on the shaft for rotation therewithabout the axis thereof, each drive ring is an annular sleeve which is arotatable sliding fit around the rim of the associated torque lobe, andthe axis of each piston being offset with respect to the center of theoutput shaft whereby rectilinear movement of the associated piston isconverted to rotary movement of the associated torque lobe or viceversa, a\each connecting rod being constituted by a main connecting rodand at least one auxiliary connecting rod, the or each auxiliaryconnecting rod being slidably fixed to the associated main connectingrod for axial movement relative thereto, each piston being fixed to theor each associated auxiliary connecting rod, and each main connectingrod being fixed to the associated drive ring, and wherein the or eachauxiliary connecting rod is associated with a respective cam fixed tothe associated torque lobe.